The present invention relates to a method of forming a cover sheet for closing an opening in a case of a rotary electric device which contains a rotary mechanism.
To operate the rotary mechanism, a screw driver or similar tool is inserted into the opening to rotate. However, during assembly of the electric device, the opening is protected by the cover sheet. Examples of rotary electric devices are trimmer capacitors, variable resistors, variable coils and rotary switches etc., The present invention may be applied to any type of such rotary electric devices.
A rotary electric device comprises a rotary mechanism portion including a rotor means and a stator means contained in a case. In such a rotary electric device, the rotor means is rotated to change, e.g., the capacity in a trimmer capacitor and the resistance value in a variable resistor. Therefore, the case of the rotary electric device is provided with an opening through which the rotor means is rotated for performing the desired adjustment to change the characteristics of the rotary electric device from the exterior thereof.
When a rotary electric device is mounted on a printed circuit board etc., flux used in soldering the device to the circuit board may enter the case through the opening formed therein. This has a negative influence on the characteristics of the rotary electric device. Further, to improve workability and reduce mass production cost, it is desired to pass all of electric or electronic components through a molten solder tank thereby to simultaneously solder a plurality of electric or electronic components on the printed circuit board etc. In this case, however, the solder inevitably enters the rotary electric device through the opening formed in the case to disadvantageously influence the rotor means and stator means etc., and as a result, for example, rotation of the rotor means might be prevented and characteristics of a dielectric member or resistance film serving as the stator means might be degraded. Thus, it has generally been required to separately solder a rotary electric device such as a trimmer capacitor after the dipping operation.
In this regard, there has been provided the so-called closed type rotary electric device to which a cover sheet is applied to close the opening defined in its case thereby to prevent entry of flux used in soldering and solder from a molten solder tank. Such a closed type rotary electric device is disclosed, for example, in Japanese patent laid open document No. 72353/1983 of the present invention and others, published on Apr. 30, 1983. In this prior art, a heat-resistant cover sheet of polyimide resin is applied to the outer surface of the case to close an opening defined therein by a heat-resistant adhesive. In the closed type rotary electric device, therefore, the entry of flux employed in soldering is prevented as is the entry of solder when the device is dipped in a molten solder tank, to thereby enable simultaneous soldering of the rotary electric device with other electric or electronic components.
The cover sheet is made of material having such thickness and strength that it can be easily broken by a screwdriver etc., and hence the rotor means can be readily rotated by the screwdriver etc. with the cover sheet being broken by the screwdriver when it is necessary to rotate the rotor means of the rotary electric device.
However, as taught by the prior art, it is difficult to perform the step of applying the cover sheet to the case of the rotary electric device. When a plurality of cover sheets are applied one by one to small-sized rotary electric devices, some as small as several millimeters square in dimension, the sizes of the rotary electric devices and the cover sheets are too small to process and handle.
Therefore, it is extremely difficult to apply such cover sheets to such rotary electric devices. Accordingly, it has been proposed to prepare a large-sized cover sheet and to apply it to a plurality of rotary electric devices. Then the larged-sized cover sheet member is cut leaving an individual cover for each device. However, it is difficult to sever the large cover sheet member along the outer periphery of the case or housing of each rotary electric device. Further, the thickness or the height of the rotary electric device is increased by application of the cover sheet, and such increase in size is undesirable particularly in small-sized rotary electric devices. Further, when a cover sheet of polyimide resin is broken while a rotor is rotated, the fragments of the cover sheet may enter the case.